Process for the partial pyrolysis of wood and product of the same



'Oct. 17, 1944.

A. 'H. WHITE PROCESS FOR THE PARTIAL PYROLYSIS OF WOOD AND PRODUCT OF THE SAME Filed Oct. 7, 1940 FIG.II.

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)fll l kl z ee*$,-+ K V C Pm-f/a/ Condenser M I Condenserl. l 1 P H" I I J Ill/ll IIII 1| I uron'r E 7 n f l'/ /4////////// ////M ZN NTOR. ALFRED H.WHITE "ATTOR EYS the Patent ed Oct. 17, 1944 PROCESS FOR THE PARTIAL PYROLYSIS OF WOOD AND PRODUCT OF THE SAME Alfred H. White, Ann Arbor, Mich. Application October '7, 1940, Serial No. 360,203

7 Claims.

The invention relates to the manufacture of products by pyrolysis of wood and has for its object the obtaining of a process in which the wood under treatment is not completely destroyed or reduced to charcoal but retains a fairly high percentage of its original structural strength. It is a further object to obtain a modified wood product having improvedcharacteristics for certain uses. Still further, it is an object to obtain other products by partial pyrolysis of the wood which, while not as great in quantity as would result from complete pyrolysis, are still suflicient to be of value. With these objects in view, the invention consists in the process and products thereof as hereinafter set forth.

In the drawing:

Figure 1 is a diagrammatic illustration of an apparatus suitable for carrying out my improved process;

Figure 2 is a similar view showing a modified construction Figure 3 is an enlarged section of a portion of the retort shown in Figure 2.

The pyrolysis of wood-in retorts to form charcoal with recovery of methanol, acetic acid and other by-products has been practiced for almost a hundred years. Theoretical studies of the pyrolysis of'wood were made by Violette in 1851, Klason in 1909 and by many other investigators, notably Hawley and his colleagues at the Forest Products Laboratories.

There is general agreementthat when wood is heated in a closed retort to about 280 C. an exothermicreaction occurs which causes the temperature to rise rapidly, with formation oi charcoal and evolution of methanol, acetic acid and other by-products. All of the commercial methods for the pyrolysis of wood have permitted the'temperatures to rise to a point where this exothermic reaction took place with formation of charcoal.

Hawley in his book on Wood Distillation-sumon the yields.

It has been recognized that moisture in the wood produces steam which may influence the reaction, but there is no agreement as to whether the steam exerts an important effect on the products formed. Hawley states on page 58 of the reference already cited:

It is now possible to speculate on the possible efiect of varying moisture content of the wood Palmer has made a study of this question in small scale apparatus and found that the effect varied with the species and with the speed of distillation so that no general conclusions can be drawn.

The process which I have invented deals with the partial pyrolysis of wood in an atmosphere which contains a large amount of steam, and in which the temperature is controlled so that there is no pronounced exothermic reaction and the final solid product is modified wood instead of charcoal.

The results obtained by my process will be illustrated by the following data from a single run.

The charge consisted of nine blocks of oak wood each 1" x 1" -x 4" which had been dried at 105 C. prior to the test. These were placed in a steelv to flow slowly through the retort. The temperature rose to 240 C. in about 2 hours more and was maintained at that level for 10 hours. The temperature was measuredby two thermocouples, one embedded in one of the wooden blocks and the other, in the vapor space. These two couples difiered very little from each other throughout the whole run. Atmospheric pres; sure was maintained throughout the test. The steam and gases evolved were passed through a condensing system and the liquid distillate was collected. In this test the'weight of steam intromarizes theexperiments of'earlier investigators who had studied the pyrolysis of wood and states,

- page 54:

ited to the conditions of the experiment in which So far as actual the heat is applied rapidly. destructive distillation is concerned, then, the reaction begins about 250 0.,soon becomes exothermic and is finished at about 350 C.

" duced was 2.4 times the weight of the wood. At

noted that the-treated-wood has a compressive strength (when the pressure is applied to the end of the grain) of '1090. pounds per square inch 2 v a I 2,860,604

- made the woodconsiderably more brittle. It was also found that the treated wood could readily be pounded to small pieces, and then ground to a fine powder. The treated wood was also more porous, but absorbed a smaller amount of water and expanded less when kept in a damp box at 25 C. until equilibrium was reached, as shown in Table I.

Chemical analysis showed that almost 94 per cent the pentosans and 35 per cent of the cellulose had been-decomposed by'the heat treatment, while the lignin showed an apparent increase. The acetic acid obtained by hydrolysis of the wood dropped from 4.0 in the untreated wood to 0.2

to 400 C. with production of charcoal. The results of this second column correspond more closely to those obtained in commercial operation, although steam was introduced in a similar manner in both tests. The partial pyrolysi at 240 0. caused the evolution of 78 per cent as much acid, calculated as acetic, and of 62 per cent as much furiural, but no measurable amount of methanol.

Other experiments conducted insa similar manner showed that wood was decomposed to a lesser extent at as low a temperature as 180 C. A test at 180 0. showed that the dried wood had lost not only as an inert gas but also is active in cansing hydrolysis of the organic compounds.

TABLE I Properties of oak wood before and after partial pyrolysis Alter Dried at treatment 105 C. in steam t 240? C Relative weight. 100 74.7 Relative length (tangential) 100 92. 4 Relative apparent density 100 85. 6 Crushing strength ounds per sq. in.) 8, 250 7,090 After exposure to amp air at C. until equilibrium was reached:

Per cent moisture absorbed 20. 3 B. 9 Per cent increase in length 6. 4 2. 3 Chemcal composition in per cent of dried woo Total pentosans 20.6 1 2 Cellulose (Cross and Bevan) 55.4 36 4 Lignin 2s. 7 s4 7 Methoxyl content 5. 9 4 2 Acetic acid by hydrolysis 4. 0 0 2 6.5 per cent of its weight and that acid, titrated tests on birch and on other woods have shown "similar results, and there is reason to believe that all types of woods and vegetable tissues will respond to this treatment in a similar manner although the upper and lower permissible temperatures will vary somewhat with the different species.

Inthe example which has been given dried wood was heated in an atmosphere of steam. It was not necessary to use dried wood and some of the steam might equally well have been generated from the moisture in the wood. It is recognized that steam is always present in the commercial pyrolysis of wood, but the amount becomes low in the later stages of the commercial process as the wood becomes dried and as gas is evolved through the pyrolysis of the wood. In my process, the steam has its greatest value at temperatures above 220? 0. when there is danger that arr exothermic reaction will start and cause the temperature to rise in an uncontrolled manner.'

There is always some gas in the retort in my process but I aim to keep the volumeof steam at least j prevent an exothermic reaction from gainin headway and at temperatures below that at which noticeable charring occurs. The steam acts Tllnu: II

Products of partial pyrolysis of oak wood [Expressed in per cent by weight of dried wood] After treatment in steam- At 240' C. At 400 0. maximum maximum Various forms of apparatus may be used ior carrying out my improved process but, as diagrammatically illustrated in Figure 1, A is aretort of any suitable constructionror containing the wood B to be treated. This retort is placed within a iurnace C shown as an electrical turnace having a heating resistor D. E is a conduit connected to one end of the retort and controlled by a valve F which regulates the admission of a constant stream 01' steam. G is a conduit leading from the opposite end of the retort passing through a condenser H, the condensate being collected in a receptacle I while the uncondensed gases escape through a vent J.

In Figure 2, a modified construction is shown which, in addition to the elements above described, has a return conduit K leading from the receptacle I to the conduit E and controlled by a valve L. A blower or other propelling formed by pyrolysis and or the steam condense and are collected in the receptacle I. A portion of the steam and of the gases and vapors formed in the retort are returned by the blower M with a resultant sawing in i'uel, and a higher concentration of valuable products in the receiver than would be thev case if no steam 'were'recirculated.

Although the modified wood has become morev brittle through the heat treatment, it may still be shaped by'ordinary wood-working methods.

Pieces of wood may be given approximate final shape before placing in the retort, since the shrinkage within the retort is uniform and no subsequent drying or curing operation is necessary. The lower density of the modified wood and the smaller volume changes which it undergoes when subjected to changes in atmospheric humidity make the product valuable forcabinet making. Its greater porosity increases its value as a heat insulator. When thin slabs of wood or veneers are given this treatment, the resultant modified wood lends itself to a stable laminated product because of the ready absorption of the adhesive and the ability of the wood to withstand short exposures to 250 C. in the curing press without injury.

The modified wood is less liable to decay and attack by insects because of the low water, absorption in damp air and because the heat treatment has destroyed most of the pentosans and similar products which furnish the most available food for many varieties of molds, bacteria and insects.

The color of the modified wood has been darkened uniformly throughout the cross section of the piece by the heat treatment, so that a piece, after heat treatment, may be turned or planed without the development of inequalities in color.

The wood may be introduced into the retort in the form of veneers, chips or sawdust. These pieces of small cross section require only a relatively short time in the retort, and may be heated to a somewhat higher final temperature than the larger pieces. Sawdust and chips are especially suitable forms of raw material when wood flour is to be the final product, since the heattreated product pulverizes readily. The resultant wood flooris especially adapted for incorporation in phenolic resins and other plastics, because of the porosity, rigidity and small volume changes of the particles with variations in atmospheric humidity, and their stability at curing temperatures somewhat above 250 C.

What I claim as my invention is:

1. The process for producing a partial pyrolysis of wood which comprises externally applying heat to the wood in a retort thereby raising the temperature of thewood to above 180 C., continuously flowing through said retort. when the temperature of the wood has reached 150 to 180 0., an atmosphere containing at least 90% of steam at a pressure not substantially above atmospheric thereby continuously removing the volatile products liberated from the wood and also thereby preventing a temperature rise above 275 C. due to exothermic reaction, and maintaining the temperature of said wood between 180-275 C. for a time suflicient to produce at least 2% of acids calculated as aceticacid on weight of dry wood with no appreciable amount of methanol and to reduce the pentosans by at least 50% and cellulose at least 20% of the dry weight of said wood while retaining the lignins and at least 50% of the compressive strength of the wood.

2. The' process according to claim 1 in which the continuously flowing atmosphere is superheated steam 3. The process for producing a partial pyrolysis of wood which comprises externally applying heat to the wood in a retort thereby raising the temperature of the wood to approximately 240 C., continuously flowing through said retort, when the temperature of the wood has reached not more than 220 C., an atmosphere containing at least 90% of steam at a pressure not substantially above atmospheric thereby continuously removing the volatile products liberated from the Wood and also thereby preventing a temperature rise above 275 C. due to exothermic reaction, and maintaining the temperature of said wood at approximately 240 C. for a time suiiicient to produce at least 2% of acids calculated as acetic acid on weight of dry wood with no appreciable amount of methanol and to reduce pentosans by over and cellulose at least 20% of the dry weight of said wood while retaining the lignins and at least 25% of the compressive strength of the wood.

4. The process according to claim 3 in which 7 the continuously flowing atmosphere is superheated steam.

5. The process for producing a partial pyrolysis of wood which comprises externally applying heat to the wood in a retort thereby raising the temperature of the wood to approximately 240 C.,

continuously flowing through said retort, when the temperature of the wood has reached not more than 220 C., superheated steam at substantially atmospheric pressure thereby continuously removing the volatile products liberated from the wood and also thereby preventing a I temperature rise above 275 C. due to exothermic reaction, and maintaining the temperature of said wood at approximately 240 C. for a time on the order of ten hours.

6. The process for producing a partial pyrolysis of wood which comprises externally applying heat to the wood in a retort thereby raising the temperature of the wood to above 180 C., continuously flowing through said retort, when the temperature of the wood has reached 150 to 180 C., an atmosphere-containing at least 90% of steam at a pressure not substantially above atmospheric thereby continuously removing the volatile products liberated from the wood and also thereby preventing a temperature rise above 275 0., due to exothermic reaction, maintaining the temperature of said wood between 180-275 C. for a time suflicient to produce at least 2% of acids calculated as acetic acid on weight of dry wood with no appreciable amount of methanol and to reduce the pentosans by at least 50% and cellulose at least 20% of the dry weight of said wood While retaining the lignins and at least 50% of the compressive strength of the wood, condensing out certain of said volatile products from said atmosphere at a temperature not substantially below the boiling point of water, recirculating a portion of the atmosphere containing uncon densed steam through said retort and condensing out from the remaining portion of said at- .mosphere the condensable volatilized products from said wood,

7. A modified form of wood produced by the partial pyrolysis of natural wood, free from the volatile products of decomposition from said partial pyrolysis, having less pentosans and 20% less cellulose than the original wood, containing asmuch lignin as the original wood, having at least 50% of the original compressive strength, having a higher resistance to decay and characterized by substantially smaller volume changes when exposed to humid conditions than the original wood.

ALFRED H. WHITE. 

